Conventional microlithographic exposure apparatus employing a charged-particle beam such as an electron beam comprise a charged-particle-beam (CPB) optical system for directing the charged-particle beam passing through a mask onto the surface of a sensitive substrate such as a semiconductor wafer. Conventional microlithographic exposure apparatus typically comprise any of various measurement subsystems used for making, inter alia, accurate positional measurements (e.g., of the mask and wafer relative to each other) and measurements of the optical performance of the CPB optical system. Making such measurements is typically facilitated by fiducial (reference) marks that serve as points of reference. By way of example, an electron-beam exposure apparatus typically directs an electron beam to irradiate a fiducial mark situated on a wafer stage and detects electrons reflected from the fiducial mark.
Typical fiducial marks comprise a pattern of rectangular "voids," spaced apart from one another at a selected pitch on a mark substrate. Electrons from the electron beam are reflected from, e.g., regions of the mark situated between the voids, and pass through the voids.
Conventional fiducial marks include marks etched in silicon and marks patterned using a heavy metal. Marks etched in silicon are generally formed by applying a resist to a silicon substrate, inscribing a pattern into the resist using an electron beam to form a resist mask, and selectively etching the silicon substrate using the resist mask. Marks patterned using a heavy metal are generally formed by depositing, in a desired pattern, a heavy metal layer on a suitable substrate; by using a resist mask as described above and etching, a pattern is formed in the heavy metal layer.
One drawback of conventional methods for forming fiducial marks is that such methods produce mark patterns having imperfect edges. The magnitude of edge imperfection, i.e., edge "roughness," is about 10 nm in resist patterns formed by electron-beam inscription. Such edge roughness is amplified during subsequent etching or deposition of heavy metal; the resulting fiducial marks have pattern edge roughness of about 20 nm.
The precision of aberration measurements corresponds to the accuracy of the fiducial mark used in making the measurements. The magnitude of edge roughness in marks produced by either of the foregoing methods adversely affects the detection signal, which decreases the accuracy of the aberration measurements. Also, a charged-particle beam having a large-area transverse section is typically used in CPB projection-exposure apparatus; such a beam causes the signal-to-noise (S/N) ratio of aberration measurements performed using a heavy-metal fiducial mark to be reduced due to background noise from the mark substrate.